Extensive use has been recently made of apparatus for drying and calcinating coated welding electrodes, which are provided with a means for conveying the electrodes through a heating chamber and in which the thermal treatment of the electrodes may be implemented by a variety of methods, for example, by supplying heated air into the above chamber from air heaters, by using electric heaters arranged above or under the supporting element of the conveying means, by infra-red or radiant heating, and so forth. Furthermore, for supplying heat to the welding electrodes being thermally treated, use may be made of different combinations involving the foregoing and other methods.
One of such apparatus available in the prior art (see, for example, I. I. Garnik et al., The Production of Metal Electrodes, Moscow, "Metallurgia" Publishing House, 1975, p. 88-89) comprises a heating chamber having a conveying means arranged thereinside which incorporates horizontal chain conveyers arranged in a number of tiers in height and furnished with side plates secured to their chains, and which is provided with arrangements for loading and unloading the electrodes. The loading arrangement of the apparatus comprises a tray, wherefrom the electrodes to be thermally treated are delivered to a receiving conveyer of the multitier conveying means. The unloading arrangement comprises a plurality of flexible supporting and pulling members. As a heat supplying means, serve tubular electric heating elements mounted inside the heating chamber, disposed in the spaces between the chain conveyers and arranged perpendicularly to the direction of travelling of the runs of the aforesaid conveyers.
In the above-described apparatus the members of each of the chain conveyers, in particular, their bush-roller chains, are located immediately in the zone of the intensive thermal action applied thereto by the electric heating elements. Therefore, as long as the apparatus is operating, the bush-roller chains stay in a heated condition to a temperature which is close to the working temperature of the apparatus. Under these circumstances the bush-roller chains can not function normally for a long time, which results in their quick wearing and subsequent failure.
In the above apparatus, the coated welding electrodes subjected to the thermal treatment may be arranged on the chain conveyer in a few layers in height and while being conveyed they tend to come in contact with one another as well as with the other elements of the conveyer. This may cause the electrodes to stick together and result in the development of embedments in their coating, as well as in the abrasion of and damages to the coating. The coating of the electrodes may be also damaged while they are transferred from the conveyor of one tier of the conveying means to the conveyer of its other tier. However, when the welding electrode has a damaged coating, it is usually discarded.
Furthermore, due to the use of the convection technique of heat supply, which is characterized mainly by the external heating of the coating of the welding electrodes being thermally treated, it is difficult to ensure simultaneously both a high quality of the coating of the finished welding electrodes and a high output of the apparatus. This is explained by the fact that the coating of the welding electrode heated externally is warmed irregularly in its layer thickness, insofar as the most humid portion of this layer, which is adjacent directly to the metal rod of the electrode, is heated in the last place. Therefore, in order to provide the proper conditions for heating of the coating of the welding electrode there is required a comparatively extended period of time. If, however, the process of heating is accelerated by this or that means, then the coating of the electrode may develop fractures, deteriorating the mechanical strength and moisture permeability of the coating, that is, in the final analysis, its quality.
The best characteristics of heating are offered by the induction method of heat supply, in which the coating of the welding electrode is heated from the inside due to the excitation of strong eddy currents in the rod of the electrode warming this rod. As a result of heating the rod in this manner, the coating of the electrode is heated from the inside, which contributes to the more intensive removal of moisture from the coating and to a considerable increase in the rate of drying, the rate of drying being accelerated, in a general case, from 3 to 12 times.
Also known in the prior art are apparatus for drying and calcinating coated welding electrodes using the induction method of heating. One of such apparatus for drying and calcinating coated welding electrodes with the use of induction heating (see, for example, F.R.G. Pat. No. 1565293 issued Mar. 30, 1972), which is closely related in its technical essence to the proposed apparatus, comprises a chamber having a slot induction heater arranged thereinside which is designed for heating the welding electrodes and provided with a chain conveyer designed for conveying the electrodes being thermally treated along the length of the horizontally arranged slot of the induction heater and furnished further with an arrangement for loading the electrodes with a raw coating thereinto and an arrangement for unloading therefrom the electrodes with a thermally treated coating.
The slot induction heater of the above prior art apparatus has a zigzag shape and designed as a sectionalized structure in length. Each of its sections is made up from a few bus-bars arranged in parallel to which an electric current of the same magnitude is applied and which have a different width, as a result of which the density of the current flowing through the side electroconductive bus-bars is one and a half times as much as the density of the current flowing through the middle electroconductive bus-bars. The chain conveyer, also having a zigzag shape reproducing the shape of the slot induction heater, comprises two endless double-strand chains of the bush-roller type arranged in parallel and provided with a plurality of pairs of clamping holders, which may be designed also as magnetic holders, one chain being fitted with stationary clamping elements of each of the pairs of the electrode holders, while the other chain is fitted with spring clamping elements of the aforesaid pairs, and each of these elements embracing one of the ends of the electrode being held. Each of the arrangements for loading and unloading disposed in the terminal portions of the chain conveyer represents a drum having transverse grooves for receiving the coated welding electrodes, the drum of the unloading arrangement being provided with a means for removing the thermally treated welding electrodes from the chain conveyer.
In this apparatus the holders of the electrodes of the chain conveyer are located, while the electrodes are passing through the slot induction heater, in the space of the slot of the heater, that is, in the zone of action of the highest temperatures created thereby, while the chains of the conveyer are located in direct proximity to the foregoing zone. Consequently, as in the case of the above-described apparatus, the elements of the chain conveyer and, in particular, the electrode holders are subjected to the continuous action of high temperatures deteriorating their operating conditions and reducing considerably their service life. Also subjected to the severe temperature action is the hygroscopic material that must be used in the adopted construction of the holders for removing moisture from the coated end of the electrodes embraced by the holder for the purpose of ensuring the proper thermal treatment of this portion. However, it is known that the choice of available temperature-resistant hygroscopic materials is fairly limited.
Furthermore, the location of the electrode holders directly in the slot of the induction heater calls for an increase in the dimensions of the slot, on the one hand, in terms of its width, which requires an unjustified built-up in the consumption of power for achieving the desired temperature of heating, and, on the other hand, in terms of its length, which with the assigned number of the electrodes being simultaneously treated in the apparatus dictates an increase in the overall dimensions of the slot induction heater, and, hence, of the entire apparatus.
While conveying the coated welding electrodes through the slot induction heater, the electrode holders may cause severe abrasion of the surface of the current-feeding bus-bars of the induction heater, which are located in direct proximity thereto. Also subjected to substantial abrasion may be the coating at the ends of the electrodes being conveyed and embraced by the holders. This abrasion results in the formation of significant amounts of dust in the apparatus which may settle down together with the products of wearing of the hygroscopic material of the holders on the coating of the electrodes being thermally treated. The dust including metal and other materials and deposited and thermally consolidated on the coating of the welding electrodes impairs, on the one hand, the commercial aspect of the finished welding electrodes, and, on the other hand, may result in spoilage of the welds performed by these electrodes.
It should be also noted that the construction of the chain conveyer consisting of the two parallel-arranged chains between which are located the welding electrodes being conveyed compels supplying to the foregoing conveyer the electrodes, of a stringently pre-assigned length with a relatively small tolerance. Consequently, this may call for the incorporation of an additional appliance into the apparatus for grading the electrodes being delivered to the conveyer to their length.
Finally, should even a single electrode fall by chance out of the holders in the apparatus under consideration, it may lead, due to the horizontal arrangement of the slot of the induction heater, to the collapse of many electrodes, which is accompanied by rejection of these electrodes, as well as by short-circuiting of the electroconductive bus-bars of the induction heater and considerable damage to these bus-bars, which results in shutting down the apparatus and calls for replacing the damaged bus-bars.